1. Field of the Invention
The present invention relates to a process for producing a circuit substrate which is useful for forming a precise wiring pattern on a ceramic substrate.
2. Description of the Related Art
Conventionally electronic equipment has been provided with built-in electronic components. The electronic components are formed of ceramic substrates which can be fired at relatively low temperatures ranging from 800.degree. C. to 1000.degree. C. (Japanese Publication No: 53269/1991, U.S. Pat. No. 4,621,066.
A conventional ceramic circuit substrate is produced by a process in which as a first step via holes and wiring patterns are formed in a plurality of ceramic greensheets. Thereafter, as FIG. 18 shows, ceramic greensheets 71-74 are stacked and sandwiched between plates 11 and 19 which are positioned parallel to each other. The ceramic greensheets are compressed by applying downward pressure from upper plate 11, and are affixed together into a ceramic compressed body. The ceramic compressed body is fired in order to prepare a ceramic circuit substrate.
The ceramic circuit substrate which is prepared, however, has a substantially large planar dimensional tolerance exceeding .+-.0.3% at compressing and firing, because of spread both in the planar elongation rate and in the dimension of the greensheets at compressing, and differences in shrinkage of the individual greensheets at firing. These factors significantly complicate the precision of positioning the surface wiring pattern on the greensheets.
Recently improvements in the planar dimensional precision of ceramic circuit substrates have been required by tightening the tolerance to .+-.0.05% or less because of the necessity of current technological tendencies such as down-sizing and high densification.
Conventional circuit substrates of other types are produced by preparing greensheets in a first step which are used for forming ceramic bodies containing glass. They are provided with holes which are filled with conductors. Wiring patterns are printed on the greensheets. A plurality of greensheets having the wiring patterns formed thereon are prepared. Those greensheets are stacked, compressed, and fired at a temperature ranging from 800.degree. C. to 1000.degree. C. thus providing circuit substrates. However, in this process for producing the conventional circuit substrate, as FIG. 19 shows, when stacking, compressing, and firing to obtain a plurality of ceramic substrates 99 and 98, the irregularity of the wiring patterns 59 may cause unevenness of the ceramic substrate 98. In this case, as FIG. 20 shows, when forming a wiring pattern 58 on uneven ceramic substrate 98, the width of the wiring pattern 58 may deviate from that of the designed pattern 580 thereby causing the tolerance to exceed .+-.10 .mu.m because of the unevenness of the wiring pattern 59. The large dimensional tolerance may cause a blur 581, deficiency 582, or the like in the resultant pattern.
In the event the wiring pattern is printed on the ceramic greensheet of a top, as shown in FIGS. 21 and 22, protuberances 571 of conductors formed by filling holes 90 with conductive material may cause blurs 581 and deficiencies 182 in the wiring pattern 58. In the event the wiring pattern is provided with a resistor, the tolerance of the resistance value may exceed .+-.25%, resulting in a decrease in the yield of resistance trimming.
Alternatively the circuit substrate may be produced by using a pre-fired ceramic body obtained by the preliminary firing of the ceramic greensheets. In this case, the solvent present in the paste which forms the wiring pattern on the surface of the pre-fired ceramic body is not absorbed, which means that the wiring pattern is not formed precisely.